What Is Speed?
Speed is the rate at which an object covers distance. It tells you how fast something is moving, without regard to direction. A car driving at 100 km/h has the same speed whether it heads north or south.
Speed is a scalar quantity: it has magnitude only. This distinguishes it from velocity, which is a vector (magnitude + direction). The SI unit of speed is the metre per second (m/s).
Speed Formula
Speed equals distance divided by time. This is the fundamental relationship connecting how far an object travels with how long it takes. Rearranging gives d = s × t and t = d / s.
Distance\u2013Time Graph
On a distance–time graph, speed is the slope of the line. A steeper slope means higher speed. A horizontal line means the object is stationary (speed = 0).
Speed vs Velocity
| Property | Speed | Velocity |
|---|---|---|
| Type | Scalar | Vector |
| Has direction? | No | Yes |
| Uses | Distance | Displacement |
| Can be negative? | No (≥ 0) | Yes |
| Example | 60 km/h | 60 km/h north |
Average Speed vs Instantaneous Speed
Average Speed
Instantaneous Speed
Average speed considers the entire trip: total distance / total time. Instantaneous speed is the speed at a single moment (what a speedometer reads). For constant speed, they are equal.
Uniform Motion
Uniform motion means an object moves at constant speed in a straight line with zero acceleration. In uniform motion:
- Speed = constant at every instant.
- The distance–time graph is a straight line (slope = speed).
- Average speed = instantaneous speed.
- Distance and displacement are equal (straight-line path).
- No net force acts on the object (Newton’s first law).
Real-world examples of near-uniform motion: cruise control on a highway, a satellite in circular orbit (constant speed, though direction changes), a conveyor belt.
Speed Unit Conversion Table
| From → To | m/s | km/h | mph | knots |
|---|---|---|---|---|
| 1 m/s | 1 | 3.6 | 2.237 | 1.944 |
| 1 km/h | 0.2778 | 1 | 0.6214 | 0.5400 |
| 1 mph | 0.4470 | 1.6093 | 1 | 0.8690 |
| 1 knot | 0.5144 | 1.852 | 1.1508 | 1 |
Real-World Speed Reference
| Object | m/s | km/h | mph |
|---|---|---|---|
| Walking | 1.4 | 5 | 3.1 |
| Running (jogging) | 2.8 | 10 | 6.2 |
| Usain Bolt (peak) | 12.4 | 44.7 | 27.8 |
| Cycling | 6.9 | 25 | 15.5 |
| Car (highway) | 27.8 | 100 | 62.1 |
| High-speed rail (TGV) | 83.3 | 300 | 186 |
| Commercial jet | 250 | 900 | 559 |
| Sound in air (20°C) | 343 | 1,235 | 767 |
| SR-71 Blackbird | 1,020 | 3,675 | 2,284 |
| ISS (orbital) | 7,660 | 27,600 | 17,100 |
| Light in vacuum | 299,792,458 | 1.08 × 10&sup9; | 6.71 × 10&sup8; |
How to Use the Calculator
- Select a mode: speed from distance/time, distance from speed/time, average speed, two-leg trip, relative speed, or unit conversion.
- Enter known values and select units.
- Click Calculate.
- Review the result, step-by-step solution, and unit conversions.
Example Calculations
100 m in 10 s
s = 100 / 10 = 10 m/s = 36 km/h
Marathon: 42.195 km in 2 h
s = 42,195 / 7,200 = 5.86 m/s = 21.1 km/h
Car: 100 km/h for 3 hours
d = 27.78 × 10,800 = 300,000 m = 300 km
Two-leg: 60 km at 60 km/h + 60 km at 120 km/h
Total: 120 km in 1.5 h = 80 km/h (not 90 km/h)
Common Mistakes
- Averaging speeds arithmetically instead of using total distance / total time.
- Confusing speed (scalar) with velocity (vector).
- Mixing units (e.g., distance in km with time in seconds without converting).
- Using displacement instead of distance for speed calculations.
- Forgetting that speed is always ≥ 0.
Accuracy and Limitations
This calculator assumes constant speed for basic modes. Real-world motion involves variable speed due to acceleration, friction, and other forces. The Mach number conversion uses sea-level standard atmosphere (343 m/s); actual speed of sound varies with altitude and temperature. For relativistic speeds (approaching light speed), use special relativity formulas. This tool is for education and estimation.
FAQ
What is speed in physics?›
Speed is the rate at which an object covers distance. It is a scalar quantity (magnitude only, no direction) measured in metres per second (m/s) in SI units. Speed = distance / time.
What is the difference between speed and velocity?›
Speed is a scalar — it tells you how fast something moves regardless of direction. Velocity is a vector — it includes both magnitude and direction. An object moving in a circle at constant speed has changing velocity because its direction changes.
How do you calculate average speed?›
Average speed = total distance / total time. For a two-leg journey at different speeds, you cannot simply average the two speeds. You must find the total distance and total time first.
Why is average speed not the average of two speeds?›
Because you spend different amounts of time at each speed. If you drive 60 km at 60 km/h (1 hour) and 60 km at 120 km/h (0.5 hours), the average is 120 km / 1.5 h = 80 km/h, not (60+120)/2 = 90 km/h.
How do I convert km/h to m/s?›
Divide by 3.6. For example, 100 km/h = 100 / 3.6 ≈ 27.78 m/s. To go the other way, multiply m/s by 3.6.
How do I convert mph to km/h?›
Multiply by 1.60934. For example, 60 mph = 60 × 1.60934 ≈ 96.56 km/h.
What is instantaneous speed?›
Instantaneous speed is the speed at a specific moment in time. It is the magnitude of the instantaneous velocity. A car’s speedometer shows instantaneous speed.
Can speed be negative?›
No. Speed is the magnitude of velocity and is always ≥ 0. Velocity can be negative (indicating direction), but speed cannot.
What is relative speed?›
Relative speed is the speed of one object as observed from another. For objects moving in the same direction, subtract speeds. For opposite directions, add speeds (in classical mechanics).
What is Mach number?›
Mach number is speed divided by the local speed of sound. Mach 1 = speed of sound (≈ 343 m/s at sea level, 20°C). Mach 2 = twice the speed of sound, etc.
Sources
Author & technical reviewer
Manish Kumar
PhysicsCalcs tools are reviewed with an educational focus: clear formulas, transparent assumptions, and practical context for students and science learners.
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